Re: [Regex] \r matching \n

On 3/25/26 10:22, Olaf van der Spek via Boost wrote:
> On Fri, Mar 20, 2026 at 8:34 PM John Maddock via Boost
> <[email protected]> wrote:
>>
>> On 20/03/2026 07:29, Olaf van der Spek via Boost wrote:
>>> In this regex the \n matches \r. How is this defined / controlled? Is
>>> there a way to only match \n?
>>>
>>>       std::string s = "[*] A\rB\n";
>>>       s = boost::regex_replace(s, boost::regex(R"(\[\*\](.+?)(\n|$))"),
>>> "<li>\\1</li>");
>>>       std::println("{}", js_encode(s)); // <li> A</li>\rB\n
>>
>> \n is a regular expression meaning "Match any newline character", which
>> happens to include \r and a few other things as well.  If you want to
>> match a literal '\n' then you need to escape it and the appropriate
>> regular expression string is therefore R"\\n"
> 
> Hi John,
> 
> Did you mean R"(\\n)" ?
> Or "\\n" ?

According to documentation 
(https://www.boost.org/doc/libs/latest/libs/regex/doc/html/boost_regex/syntax/perl_syntax.html):

R"(
)" = "\n" should match itself, like all characters not in .[{}()\*+?|^$

R"(\n)" = "\\n" should match exactly '\n' and not '\r', just like "\n". 
The documentation is explicit about this.

R"(\r)" = "\\r" should match '\r', as should "\r", for what it's worth.

R"(\\n)" = "\\n" should match "\\n", i.e. a backslash followed by 'n'.

'$' should match the end of a line, including embedded newlines in the 
text.  It is not clear what qualifies as a newline in this sense, but 
I'm guessing '\r' might qualify.

Any other behavior is either an error in the documentation or a bug in 
the code.


-- 
Rainer Deyke - [email protected]

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